leetcode Letter Combinations of a Phone Number

原题理解：https://leetcode.com/problems/letter-combinations-of-a-phone-number/

Description

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string “23”
Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].
Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.
dfs。。

class Solution {public:    vector<string> letterCombinations(string digits) {        init();        ans.clear();        n = digits.length();        if (!n) return ans;        dfs(0, "", digits);        return ans;    }private:    size_t n;    map<int, string> A;    vector<string>ans;    void init() {        A[2] = "abc";        A[3] = "def";        A[4] = "ghi";        A[5] = "jkl";        A[6] = "mno";        A[7] = "pqrs";        A[8] = "tuv";        A[9] = "wxyz";    }    void dfs(int cur, string x, string digits) {        if (cur == n) { ans.push_back(x); return; }        int d = digits[cur] - '0';        for (size_t i = 0; i < A[d].size(); i++) {            dfs(cur + 1, x + A[d][i], digits);        }    }};